1. Field of the Invention
The invention relates to a coextrusion apparatus, to a single-layer extrusion apparatus, and to a retrofit kit, as well as to a method for measuring a layer thickness, for producing a plastic film, and for retrofitting an extrusion apparatus.
2. The Prior Art
Extrusion of plastic films is common practice and has proven itself for many years. A starting material, generally a plastic granulate, is plasticized, in other words conveyed, melted, homogenized, and put under pressure, with an extruder. The resulting plastic melt is passed to the extrusion block by the extruder, by way of a feed, in which block the extrusion tool, generally in the form of a broad-slit die or a round die in the case of a blown-film method, ultimately forms a film from the plastic melt. By way of being permanently drawn off, the film, in melt form, gets into a cooling region and can solidify there, for example on a cooling roll, usually referred to as a chill roll in practice, or, in the blown-film method, by means of cooling devices such as air-cooling rings.
Frequently, the starting material, i.e., a thermoplastic plastic, can produce not only single-layer films but also multi-layer film composites.
Coextrusion adapters are usually used for producing the coating. European Patent Application NO. EP 2 332 714 A1 shows such a coextrusion adapter.
In order to measure the layer thicknesses in the composite that is produced, U.S. Pat. No. 7,858,953 B2 proposes plasticizing fluorescent nanoparticles with the remaining starting material, for example plastic granulate. Such particles can subsequently be used to calculate the layer thickness, by means of a white-light interferometer.